PEMB foundation uplift

[jeffhed]

I designed a foundation for a PEMB. I want to know peoples methods for designing the footings for uplift. I have typically designed the footing to weigh as much as the net uplift load (0.6D+(W or 0.7E)). The calculated net uplift force has a factor of safety of 1/0.6 = 1.67 in it from the reduction of the dead load. I received a plan check comment that states that with my calculations there is only a factor of safety of 1. But this is a factor of safety of 1 in regards to the net uplift force. Should we really be increasing the footing weight to maintain a factor of safety = 1.5 over the net uplift force? To me this is more like a factor of safety of 1.67*1.5 = 2.51. Not to mention we know what the actual weight of the footing will be. This is a commercial building and the city inspectors are very good in requiring construction matches the plans, especially foundations. Am I wrong? Do others design PEMB footings that weigh 1.5 times the net uplift force?

Thanks in advance for your replies

 

[jeffhed]

BA and JAE,
I appreciate the time you guys have taken to argue against my post. This was the reason I posted it in the first place. We had designed footings in the past as stated above, but were always unsure if this was actually allowed or not. My feeling was more that it violated the letter of the code no matter what I personally felt. And I have to say, after thinking about this subject all weekend and reading through the arguments in this thread a few times, I have been convinced that the weight of the footings must be multiplied by 0.6.

 

[BAretired]

jeffhed,

That is good. And if there is any chance of a high water table, you must consider the submerged weight of concrete below water table. Adding dead weight to the foundation becomes a less attractive option when that is the case.

BA

 

[a2mfk]

Somewhat related, the next time I do a building with footing sizes dominated by uplift requirements, I would consider suggesting looking into micropiles/auger piles/etc., especially if you have a large building with lots of uplift controlled footings. They could install two or four per footing, depending on your axial dead and uplift loads. I've seen a two man crew install 2-3 per hour down 60+ feet.

I know its a case by case basis but at some point it may be more economical than just large blocks of concrete in the ground that have to sit around doing nothing until the next hurricane